1. Field of the Invention
The present invention relates to a channel estimator for use in a mobile communication terminal, and more particularly to a channel estimator using an Infinite Impulse Response (IIR) filter.
2. Description of the Related Art
Generally, a channel estimator is used for estimating a channel prediction value in order to compensate for a fading channel. A 3rd Generation Partnership Project (3GPP) physical channel used in the channel estimator includes a Common Pilot Channel (CPICH). In a general case, a Primary CPICH (P-CPICH) is used, and in a case of specified purpose, a Secondary CPICH (S-CPICH) is used for a Physical Downlink Shared Channel (PDSCH) and a Dedicated Physical Channel (DPCH).
In such a configuration of the channel estimator, a Finite Impulse Response (FIR) filter or IIR filter may be used. FIG. 1 is a block diagram illustrating the configuration of the channel estimator in which the IIR filter 102 is used. Referring to FIG. 1, the channel estimator begins its operation on a frame boundary of a CPICH, and sums I and Q signals by 256 chip, which is a spreading factor of the CPICH, in an Integration and Dump (I&D) unit 100, resulting in the obtaining of a channel prediction value for the currently received input signal.
The channel estimator enables this value to pass through the IIR filter 102, and obtains a channel prediction value (Ant_out_I 108, Ant_out_Q 110) which effects the signal received by each antenna undergoes. Here, the IIR filter 102 reduces noise using the channel prediction value which effects the currently received input signal. As a result, the noise can be estimated by using the channel prediction value. Specifically, the channel prediction value can be obtained by summing the spreading factors of the I and Q signals. When the sum of the spreading factors is compared with the spreading factor without an error, the amount of noise currently occurring can be determined. For example, in the case of a communication system in which the spreading factor of the CPICH spreads the I signal or the Q signal by 256 chip, it is possible to estimate the value of noise occurring in a channel from which the current I signal or Q signal is transmitted, according to the differences between the sum and the value without an error, i.e. ‘256’.
The relation between the input and output of the IIR filter 102 can be expressed by Equation (1), and a transfer function in the IIR filter 102 can be expressed by Equation (2). The frequency-characteristic function of the IIR filter 102 can be obtained by substituting ejw with Z in Equation (2), as expressed by Equation (3):y(n)=b0·x(n)+a1·y(n−1)  (1)
                              H          ⁡                      (                          ⅇ                              j                ⁢                                                                  ⁢                w                                      )                          =                              b            0                                1            -                                          a                1                            ·                              ⅇ                                                      -                    j                                    ⁢                                                                          ⁢                  w                                                                                        (        2        )                                          H          ⁡                      (            Z            )                          =                                            Y              ⁡                              (                Z                )                                                    X              ⁡                              (                Z                )                                              =                                    b              0                                      1              -                                                a                  1                                ·                                  Z                                      -                    1                                                                                                          (        3        )            
In the conventional channel estimator using the IIR filter 102, when the rate of the mobile communication terminal is rapid, to the effect of a fading channel or a Doppler effect more frequently occurs. For example, when the mobile communication terminal passes through a region in which the fading channel or the Doppler effect exists, noise caused by the fading channel or the Doppler effect more frequently occurs. Accordingly, there is a problem in that the performance of the channel estimator can be degraded according to the moving speed of the mobile communication terminal.
In this case, in order to compensate for the degraded performance of the channel estimator, the coefficients set to the IIR filter 102, i.e. values of a1 104 and b0 106, should be suitably set depending on the moving speed of the mobile communication terminal, thereby preventing the performance degradation of the channel estimator. However, in the conventional channel estimator, since only the coefficients b0 106, a1 104 based on a preset default value are set for the IIR filter 102, there is a problem in that it is difficult to prevent the performance degradation caused by the moving speed of the mobile communication terminal.